Guidable projectiles use different forms of systems for measuring and/or calculating their current position and systems for guiding the projectile towards the target of the projectile. A collective name for these systems is GNC systems, in which GNC stands for Guidance, Navigation and Control. Knowledge of current position, with maximum possible accuracy, is relevant for calculating how the projectile must be guided in order to reach its target. In modern projectiles, one part of the GNC system is a navigation system. The navigation system often consists of a GPS receiver and an INS unit. INS, which stands for Inertial Navigation System, uses an IMU unit as sensor for measuring the dynamic change. The IMU unit, in which IMU stands for Inertial Measurement Unit, measures the accelerations and angular velocities of the projectile. In most cases, the IMU unit is constructed with three gyrsoscopes and three accelerometers. The IMU unit uses dead reckoning for position determination, that is to say that the change in position is calculated on the basis of velocity, direction of travel and time. The INS unit uses sensor data from the IMU unit to calculate the position, velocity vector and orientation of the projectile.
The GPS receiver receives positioning information from a number of GPS satellites and, on the basis of this information, calculates a position and velocity. By combining and calculating information from the GPS receiver and the IMU unit, it is possible to determine current position and velocity with high accuracy. The position and velocity of the projectile can be calculated with the GPS receiver and/or information from the IMU unit. The change in attitude and angling of the projectile, termed yaw, pitch and roll, is measured with the gyroscopes belonging to the IMU unit.
Previously known solutions can be found, for example, in U.S. Pat. No. 6,163,021 A, which describes a sensor system for rotating objects in a magnetic field, in which the sensor system provides navigation information relative to a known position. The sensor system comprises magnetic sensors and uses the earth's magnetic field as reference for calculating the rotation of the projectile. The use of the earth's magnetic field limits how the projectile can be oriented relative to the magnetic field, which results in functionality limitations for the projectile.
An example of another previously known solution can be found in U.S. Pat. No. 6,779,752 B1, which describes a guidance system without any gyroscope. The described system uses three accelerometers and a GPS receiver. One problem with the described system is that no or limited information is obtained concerning the roll angle, which means that the navigation system does not give a complete or correct position determination.
A problem with currently existing solutions according to the above-stated document U.S. Pat. No. 6,163,021 A is that a rotating projectile for determination of roll angle is assumed. A problem with currently existing solutions according to the above-stated document U.S. Pat. No. 6,779,752 B1 is that the described navigation system does not give a complete or correct position determination.
Further problems which the invention intends to solve will emerge in connection with the following detailed description of the various embodiments.